This invention relates to improvements in spring units and more particularly to a caged spring unit wherein a contained spring can be simply endowed with a required pre-load which can be precisely maintained. The invention and its benefits can be most readily comprehended by a discussion of its embodiment in a stripper spring, the use thereof in application to the stripper plate of a die, and the problems solved in the process. Therefore, it will be so described by way of illustration. It should be understood, however, that neither the form of its embodiment nor the application thereof is so limited. Such is not intended.
In a die, the stripper plate is intended to serve several important functions. These functions should include a flattening of the stock to which the die tools are applied, prior to their impact. If not properly achieved, the surface of the stock presented to the tools might be wavy and cause fracture or malfunction of the die tools. A second desired function of a stripper plate is to prevent lateral movement of the stock. Failure to achieve this will obviously result in ruin of the stock. A third desired function of the stripper plate is to strip stock from the die tools, as a die is moved from its closed to its open position following a cutting, perforating, or forming operation by the tools.
In efforts to achieve the desired functions above noted, it has been the practice in the past to heavily pre-load the stripper plate through the medium of its backing or biasing springs. In conventional practice the mount and pre-load of a stripper plate is such that the stripper plate is continuously stressed, even in the open position of the die in which it is embodied. The stress so applied creates pressure points which tend, either initially or eventually, to bow or warp the stripper plate. Moreover, when the die is open this stress is transferred to the plate hangers in direct shear. This last introduces a requirement for a considerable number of hangers in the conventional mount of a stripper plate.
Further problems encountered in the mount of stripper plates as practiced in the prior art are evidenced each time the die punches or other tools must be sharpened or adjusted as to their projected length. While a shortening of the tools, the stripper plate must be adjusted. When this occurs, the problems noted above are compounded. This is self evident since the backing or biasing springs are normally further loaded, adding to the pre-load and stressing further the stripper plate and its hangers.